1. Field of the Invention
The present invention relates to Ni (nickel)-based casting superalloys, and particularly to an Ni-based casting superalloy suitable for a cast article having an excellent high-temperature mechanical strength and an excellent high-temperature oxidation resistance and advantageously used for large size high-temperature components (such gas turbine blades) exposed to high temperature. The invention also particularly relates to a cast article from such an Ni-based casting superalloy of the invention.
2. Description of Related Art
An effective way to increase the efficiency of turbine power generators used in coal fired power plants or gas turbine power generation plants is to increase the main steam temperature in the boiler used in such a coal fired power plant or the combustion gas temperature in the gas turbine used in such a gas turbine power generation plant. For example, in recent years, there have been continued efforts to further increase the temperature of the combustion gas used for gas turbine power generators in order to further enhance the efficiency of the gas turbine power generator. In order to withstand such high temperature, high temperature components used in gas turbines are required to have a higher oxidation resistance and a greater high-temperature mechanical strength than conventional components.
Among high-temperature components used in gas turbines, gas turbine blades (rotor blades and vanes) are exposed to the severest operating environment. In order to withstand such a very severe operating environment (such as high temperature), columnar grain Ni-based superalloys (almost entirely consisting of columnar grains), which have a high-temperature mechanical strength greater than conventional Ni-based superalloys (having a conventionally obtained cast structure), have been beginning to be used for such high-temperature turbine blades. Furthermore, for aircraft engine gas turbines and some power generation gas turbines, single crystal Ni-based superalloys (almost entirely consisting of a single crystal), which have a high-temperature mechanical strength further higher than columnar grain Ni-based superalloys, are beginning to be used. As described above, single crystal Ni-based superalloys have the greatest high-temperature mechanical strength. For example, CMSX-4® (see, e.g., JP 1985-211031 A), PWA-1484 (see, e.g., JP 1986-284545 A) and Rene′ N5 (see, e.g., JP 1993-059474 A) have been developed as such an Ni-based superalloy for casting single crystal components and applied to aircraft engine gas turbines.
Beside single crystal Ni-based superalloys, columnar grain Ni-based superalloys having a further improved mechanical strength are also promising. Typical ways to increase the mechanical strength of columnar grain Ni-based superalloys include: precipitation strengthening which involves dispersing fine γ′ (gamma prime)-phase precipitates (typically an Ni3Al phase in which an Al (aluminum) site thereof is sometimes substituted by Ti (titanium), Nb (niobium) or Ta (tantalum)) in a γ-phase (Ni-based solid solution phase) matrix; solid solution strengthening which involves dissolving a solid solution strengthening element (such as Cr (chromium), Co (cobalt), Mo (molybdenum) and W (tungsten)) in the γ-phase matrix to form a solid solution; and grain boundary strengthening which involves adding a grain boundary strengthening element (such as C (carbon), B (boron), Zr (zirconium) and Hf (hafnium)). The precipitation strengthening by γ′-phases and the solid solution strengthening of the γ-phase are effective also for single crystal superalloys. However, an element for suppressing coarsening of the γ-phase matrix grains and a grain boundary strengthening element are not intentionally added to single crystal superalloys because single crystal superalloys do not actively contain any plural crystal grains or any grain boundaries.
Casting a single crystal Ni-based superalloy article is very delicate. During the single crystal growth, an undesirable crystal grain having a growth orientation angle different from the desirable orientation angle may sometimes grow due to an accidental temperature fluctuation or presence of an undesirable impurity. Hereinafter, such a grain having an undesirable growth orientation angle is referred to as a “misoriented grain” and such an undesirable growth orientation angle is referred to as a “misorientation angle”. A problem here is that presence of such a misoriented grain (and therefore presence of a grain boundary) significantly degrades a mechanical strength of the single crystal cast article because no grain boundary strengthening element is intentionally added to conventional Ni-based superalloys for casting single crystal articles. For example, when a single crystal cast article contains a misoriented grain having a misorientation angle equal to or more than 5°, the mechanical strength of the single crystal cast article drastically decreases. In the worst case scenario, during the casting operation, a solidification crack may occur along a grain boundary generated by the misoriented grain.
In order to alleviate this problem, Ni-based superalloys for casting single crystal articles containing an intentionally added grain boundary strengthening element have been developed (see, e.g., JP 1993-059473 A). However, even using such a method, the misorientation angle is limited to less than about 15° in order to assure sufficient grain boundary strength; thus, the above misoriented grain problem cannot be fully solved.
In order to take full advantages of single crystal gas turbine blades, the blade needs to be almost entirely single crystalline (or at least must not contain any misoriented grains whose orientation angle exceeds an allowable misorientation angle).
Herein, a total length of aircraft engine gas turbine blades is usually about 100 mm. During the casting of such a relatively small component, the tendency of any misoriented grain to grow is relatively small. Therefore, single crystal aircraft engine gas turbine blades can be industrially manufactured at a sufficiently high yield. In contrast, a total length of power generation gas turbine blades is as long as about 150 to 450 mm. Such a large blade is very difficult to cast in a single crystal. Therefore, single crystal power generation gas turbine blades previously could not be manufactured at an industrially acceptable yield (i.e., at a low cost).
Because of the above problem, currently, large-size high-temperature components such as power generation gas turbine blades are usually cast to have a columnar grain crystal structure by a directional solidification method. For example, CM186LC (see, e.g., JP 1991-097822 A), Rene′ 142 (see, e.g., JP 1992-153037 A) have been developed as such an Ni-based superalloy for casting columnar grain articles. According to the above disclosures, the disclosed Ni-based superalloys for casting columnar grain articles contain grain boundary strengthening elements in order to increase the bonding strengths between neighboring columnar grains, and the articles cast from the Ni-based superalloys have a high-temperature mechanical strength comparable to those of single crystal Ni-based superalloy articles.
However, even the above-described improved columnar grain Ni-based superalloy gas turbine blades have become unable to sufficiently overcome the above problem. This is because as the combustion gas temperature has been increased, the oxidation has accelerated and the thermal stress has increased, which may potentially cause a vertical crack along a columnar grain boundary.
In order to further increase the grain-to-grain bonding strength (grain boundary strength) and overall high-temperature mechanical strengths of columnar grain Ni-based superalloy articles, various techniques have been researched and developed. For example, JP 1997-272933 A discloses an Ni-based superalloy for directional solidification, the superalloy including: 0.03 to 0.20 wt. % of C; 0.004 to 0.05 wt. % of B; 1.5 wt. % or less of Hf; 0.02 wt. % or less of Zr; 1.5 to 16 wt. % of Cr; 6 wt. % or less of Mo; 2 to 12 wt. % of W; 0.1 to 9 wt. % of Re (rhenium); 2 to 12 wt. % of Ta; 4.0 wt. % or less of Nb; 4.0 to 6.5 wt. % of Al; less than 0.4 wt. % of Ti; 9 wt. % or less of Co; and 60 wt. % or more of Ni. According to this JP 1997-272933 A, the article cast from the Ni-based superalloy by a directional solidification method does not suffer any solidification cracks during the solidification, has a sufficient grain boundary strength to ensure reliability in actual use and has a great high-temperature mechanical strength.
JP 2004-197216 A discloses an Ni-based superalloy including: about 3 to about 12 wt. % of Cr; about 15 wt. % or less of Co; about 3 wt. % or less of Mo; about 3 to about 10 wt. % of W; about 6 wt. % or less of Re; about 5 to about 7 wt. % of Al; about 2 wt. % or less of Ti; about 1 wt. % or less of Fe (iron); about 2 wt. % or less of Nb; about 3 to about 12 wt. % of Ta; about 0.07 wt. % or less of C; about 0.030 to about 0.80 wt. % of Hf; about 0.10 wt. % or less of Zr; about 0.02 wt. % or less of B; about 0.0005 to about 0.050 wt. % of rare earth elements; and the balance practically Ni and inevitable impurities. According to this JP 2004-197216 A, articles cast from the Ni-based superalloy have a high oxidation resistance.
As described above, in recent years, there have been continued efforts to further increase the temperature of the combustion gas used for gas turbine power generators in order to further enhance the efficiency of the gas turbine power generator. In order to increase the combustion gas temperature, there are needed at least large-size high temperature components (such as turbine blades) that can withstand such higher-than-conventional combustion gas temperatures. Accordingly, a strong need exists for further improvement over current Ni-based superalloys (e.g., the ones disclosed in the aforementioned JP 1997-272933 A and JP 2004-197216 A). More specifically, there is needed for an Ni-based superalloy providing a far better balance among a great high-temperature mechanical strength, a high grain boundary strength and a high oxidation resistance than conventional ones.
As another problem, the Ni-based superalloys disclosed in the above JP 1997-272933 A and JP 2004-197216 A contain costly Re and/or rare earth elements. Low cost is an essential requirement for industrial products.